1. Field of the Invention
The present invention relates to a motor-driving circuit capable of controlling a rotational position of a motor in a very narrow angular range, and relates also to a wire-bonding apparatus incorporating the motor-driving circuit.
2. Description of the Related Art
In a brushless motor from which a rectifier is removed, a permanent magnet for generating magnetic fluxes is attached to the rotating shaft of a rotor, and three-phase windings are provided around the iron core of the stator.
As is well known, the torque produced by this three-phase motor is calculated as below.
Let it be assumed that the magnetic flux generated by the permanent magnet is .phi., and that the exciting currents iU, iV and iW flowing through the three-phase windings are respectively controlled to be sine-wave currents corresponding to the rotational position of the permanent magnet. In this case, the exciting currents iU, iV and iW are expressed as follows: EQU iU=Ia cos.theta.r EQU iV=Ia.cos(.theta.r-2/3.multidot..pi.) EQU iW=Ia.cos(.theta.r-4/3.multidot..pi.)
where .theta.r denotes the rotational position (angle) of the permanent magnet.
Thus, the torque produced by the motor is expressed by: EQU TM=3/2.Ks.Ia..phi.
(Ks: a constant of proportionality)
As can be understood from the above, the torque TM produced by the motor is stable; it does not vary with a change in the rotational position of the motor.
As is well known, the three-phase windings of the motor include a Y-connection winding type and a .DELTA.-connection winding type. In either type, the torque expressed by the above formula is produced, provided that the values of the exciting currents iU, iV and iW are maintained at the same values.
As mentioned above, a motor wherein a permanent magnet is provided for the rotor can be of a brushless type since it is not necessary to supply an exciting current to the rotor, as in a conventional DC servo motor wherein windings are provided for the rotor. However, a motor-driving circuit used for driving the brushless type of motor has to commutate the exciting currents iU, iV and iW to be supplied to the three-phase windings by use of transistor switches, with the rotational angular position of the rotor being constantly detected. Since, therefore, the circuit arrangement of this motor-driving circuit is inevitably complicated, the motor-driving circuit is larger in size than a conventional DC servo motor-driving circuit. In addition, the motor-driving circuit requires a higher manufacturing cost.
In some types of motors, the rotating shaft is required to rotate within a very narrow angular range. The motor employed in a bonding apparatus is a typical example of such a motor. In the motor, a tool arm is perpendicularly attached to the rotating shaft of the motor, and the tip end of the tool arm is moved within a certain limited range in accordance with minute rotation of the motor, so as to perform a bonding operation for an integrated circuit.
This type of motor need not produce uniform torque in the entire angle range (0.degree. to 360.degree.). The motor satisfies its requirements if uniform torque, an accurate rotational position, and an accurate rotating speed are attained only within a narrow angular range (e.g., the angular range of 0.degree. to .+-.5.degree.).
Therefore, if a motor requiring such limited rotation is driven by use of a motor-driving circuit designed for a complete-rotation type motor, the motor cannot be driven at high efficiency.